As the demand for structural materials capable of meeting ever higher targets for strength and thermal stability increases, the attention of materials scientists and engineers is shifting to reinforced composite materials for use in demanding applications, such as high performance aircraft, rockets, missiles, turbines, etc. Such reinforced composites typically consist of a matrix phase with a continuous or discontinuous reinforcement dispersed therein. The matrix phase can be metallic, cermet, intermetallic, polymeric, glass-ceramic, or it can be ceramic, for example.
Ceramics are assuming a prominent role in advanced composite materials. It is common knowledge that ceramics are strong, lighter than metals, and can withstand higher use temperatures than other structural materials. But ceramics have a reputation for brittleness and flaw-dependent properties. Brittleness can lead to the catastrophic failure of a ceramic part. In efforts to overcome this undesirable property, various schemes have been devised for toughening ceramic materials and making them less flaw-dependent. One technique involves adding a reinforcement phase, thereby creating reinforced ceramic matrix composites. The same principles applied to improving the performance of ceramics have been employed with other matrix materials as well.
The reinforcement phase for a composite article can be present in the form of particles, platelets, whiskers, chopped fibers, continuous fibers, fabric, etc. The addition of a reinforcement can substantially toughen the material, as measured, for example, by the work of fracture (WoF), which is directly related to the fracture resistance of the material. WoF is discussed, e.g., in "Ceramic Matrix Composites," R. Warren Ed., Chapman and Hall, Inc., New York, N.Y. 1992, pp. 188-189.
Fibrous reinforcements, such as continuous fibers and woven or non-woven fabrics, are especially effective in composite materials. The term "woven" in this application, unless clearly limited to fabrics made by weaving, should be taken to include fabrics made by knitting, braiding, etc. as well as by weaving. The current invention utilizes either woven or non-woven ceramic fabrics, or combinations thereof, as the reinforcement phase. In general, woven two- and three-dimensional ceramic fabrics can be produced from ceramic fibers by methods well known in the cloth-making art. The ceramic fibers can be continuous monofilament, a tow, yarn or roving consisting of a multitude of monofilaments, or continuous fiber tows or yarns can be made from shorter fibers and woven into fabric. Non-woven ceramic fabrics can be produced from ceramic fibers by what, in essence, is paper-making technology, also well known to those skilled in the art, and three-dimensional orthogonal non-woven fabrics technology is quite well developed.
It is necessary, for purposes of this application, to distinguish between green ceramic fabrics which contain green ceramic fiber and sintered ceramic fabrics which contain fired or sintered ceramic fiber. For purposes of this application, a "green" ceramic fabric or "green" ceramic fiber means a sinterable fabric or fiber which is in a pre-densified, or partially fired or cured state and generally also contains a binder, e.g., a flexible polymer matrix, which facilitates production of the fiber and fabric but is later removed. A "sintered" ceramic fabric or "sintered" ceramic fiber, on the other hand, means a green fabric or fiber which has been densified, generally by heating. Both types of ceramic fabric are important components of the instant invention.
In the manufacture of a ceramic fiber-reinforced composite article, a ceramic fabric can be shaped into a preform which can be fitted into a mold corresponding to the desired article. The preform in the mold, or in some cases freestanding, is infiltrated with the desired matrix phase, e.g., metal, cermet, intermetallic, polymer, glass-ceramic, or ceramic, and may be heated and/or pressed to produce the desired composite article. In some cases one or more intermediate resin or slurry infiltration steps are included before the final consolidation.
The prior art dealing with the production of composite articles reinforced with ceramic fiber generally begins with a sintered fiber.
Singh, et al., U.S. Pat. No. 4,944,904, discloses coating fibers, including woven fibers, with BN, the only specifically exemplified fiber being carbon, although others, e.g., SiC, are disclosed. The coated fiber is then further coated with a material wettable by Si, e.g., carbon, viz., pyrolytic carbon. The resulting twice-coated fiber is admixed with an "infiltration-promoting material," e.g., more carbon, which is wetted by molten silicon. The infiltration-promoting substance may also include a ceramic material, e.g., silicon carbide. The admixture, slurried in a liquid or with a resin added, is shaped into a preform or compact, e.g., by extrusion, injection molding, die pressing, isostatic pressing, or slip casting. The preform is then infiltrated with Si plus B (0.1-10 wt %). Infiltration is conducted under an inert gas, e.g., Ar, or preferably vacuum, in a carbon furnace. It is said the technique can be used for producing composite parts of simple, complex and/or hollow geometry.
Corbett, et al., U.S. Pat. No. 5,108,964, describes the production of a preform for a metal matrix composite in which a mixture of randomly oriented inorganic fibers or whiskers, a thermoplastic material, such as paraffin, and surfactants is prepared, molded to a desired shape, and heated to remove the binder, providing a shaped preform, which can then be infiltrated with the metal. The molded preform is supported during binder removal to prevent distortion, slumping, etc. This is preferably done by packing the shaped preform into a bed of finely divided inorganic powder which absorbs the organics as their melting points are reached. The invention is specifically embodied in a printed circuit board. The '964 patent is representative of prior art which describes molding a composite part from a mix which contains both a dispersed whisker or chopped fiber reinforcement phase and the matrix phase.
White, U.S. Pat. No. 5,196,120, discloses production of a candle filter, i.e., a cylindrical tube with one open end, comprising two parts, a filtering surface, and a support made from continuous ceramic fiber, e.g., aluminoborosilicate fiber. Tows of ceramic fiber are texturized and then braided or woven into cloth and formed into the desired shape. The shape is rigidized by applying a phenolic resin to the formed cloth. The rigid member is then coated with a filtering surface, i.e., chopped ceramic fiber or felt. The construction is overcoated and infiltrated with a layer of SiC by chemical vapor deposition using hydrogen and methyltrichlorosilane, during which the phenolic is carbonized, which promotes adhesion of the SiC coated filtering surface to the ceramic preform.
The techniques described in the above-cited patents, which all employ sintered fiber/fabric, are limited in application, because the bending and stretching of the sintered fiber or fabric are restricted by the stiffness, high elastic modulus, and "memory" inherent in a sintered fiber, limiting the range of complexity which can be achieved in any composite part made using the technique. In addition, the residual strain present in fabrics woven from sintered fibers decreases their strength.
Laskow, et al., U.S. Pat. No. 4,294,788, addresses this last-cited problem. A silicon carbide fiber/silicon matrix composite article is produced by first making a preform of carbon fiber or cloth and a binder such as cellulose acetate, polyester, resins, or colloidal graphite, placing the preform in a mold, which can be made of graphite, optionally adding some silicon carbide powder, and then infiltrating the preform with molten silicon. It is said the silicon reacts with the carbon fiber to produce aligned silicon carbide crystals in a silicon matrix with silicon carbide dispersed throughout the matrix. The technique is said to be useful for the production of gas turbine, aircraft engine, diesel engine, etc. parts.
The '788 patent discloses a way to produce an article in which the ceramic reinforcement is free of mechanical strain. However, the in situ formed SiC phase is really a collection of aligned silicon carbide crystals which do not have the strength and other properties desired in a reinforcing ceramic fiber or fabric. Articles produced by this technique do not exhibit the mechanical behavior of fiber-reinforced composites.
Technical challenges confronting those seeking to reach the next performance plateau in ceramic fiber-reinforced composites have been set forth by F. K. Ko in Ceram. Bull., 68, 401-414 (1989); i.e., at p. 412, "The first challenge is the question of conversion of brittle fibrous materials to textile structures. As a rule, the higher the temperature capability of the fiber the stiffer and more brittle it is. This processing difficulty with brittle fibrous structures calls for an innovative combination of materials systems such as material and geometric hybridization." The instant invention represents such an innovation.